I'm currently on 3.5 pumps of gel, having increased from 2 to 3 several months ago and then to 3.5 a few weeks ago (4 was too much). On both occasions I've let my GP know after the event so that she can increase my prescription to cover the extra.
When I saw her last week and asked her to increase it she specifically told me that if I increase to more than 4 at any point I need to go and see her to get progesterone increased. I currently have a Mirena Coil but she said I'd need to add utrogestan to this to ensure I'm adequately protected.
I'd definitely experiment yourself and find the level that addresses your symptoms, giving it time between increases to take effect, and then go and see your gp once you're happy that you're on the right dose for you.
What I didn't ask at my GP time, and I will if I increase gel again, is whether the extra progesterone is needed when my oestrogen levels are so low (clearly either not absorbing properly or being affected by the alleged/suspected change in Oestrogel). I can understand needing more progesterone if oestrogen levels are increasing but if I need more to achieve the same level then I can't see why progesterone needs increasing too

And you're right, don't apply to inner arms as too close to breast tissue . Instructions specify outer arms or mid/outer thighs.